A hydraulic lash adjuster capable of taking up lash in an engine valve train, commonly includes two major elements, a body and a plunger. Such a lash adjuster may form a pivot for a camshaft finger follower, may be driven by a cam to actuate a valve actuating member such as a push rod or rocker arm, or may act as a hydraulic element assembly in a direct acting hydraulic valve lifter. In general, the body is a cup-shaped or cylindrical member having a peripheral outer wall with closed and open ends. The plunger may have the general form of a hollow piston with a rounded end for acting as a pivot for an associated finger follower or an end formed with a socket for engaging a push rod. Alternatively, the plunger may have an open end engagable with a cooperating member of an engine valve train. Such cooperating members include a separate piston having a rounded pivot or recessed socket end, and an end wall of a direct acting cam follower shell.
The plunger is reciprocably received within the body with close clearance for controlling the leakdown of oil between the adjoining surfaces. An inner end of the plunger includes means defining a wall with a check valve controlled orifice leading from an oil reservoir within the plunger to an enclosed space between the lower plunger wall and the closed end of the body which forms a high pressure chamber. The oil reservoir is supplied from the engine through passages in or adjacent the body and plunger.
In operation, when the associated engine valve is closed and the cam follower engages the cam on its base circle, a plunger spring, in the high pressure chamber, forces the plunger outward to take up lash between the plunger and its cooperating member, and thus remove all lash from the valve train. This lowers the pressure in the high pressure chamber so that oil is drawn from the reservoir in the plunger through the check valved orifice into the high pressure chamber which is maintained full of oil. During the next valve opening cycle, the reaction force from the engine valve spring acts downwardly against the plunger, increasing pressure in the high pressure chamber and forcing some of the oil therein through clearances between the plunger and body and out of the high pressure chamber. During operation at normal engine temperatures, this oil is replaced by makeup oil from the plunger reservoir when the valve is closed on the next phase of its operating cycle.
During start up of a cold engine, oil viscosity is high and exhaust valve growth is rapid so that a hydraulic lash adjuster which uses a spring biased plunger may not provide a sufficient leakdown rate to provide a rate of shortening of the lash adjuster adequate to avoid holding the valve off its seat on the cam base circle, a condition sometimes called thermal pump up. This condition may cause improper engine operation or stalling and thus requires correction.
Mechanically lashed valve trains provide sufficient lash to accommodate transient growth of valve train components following start up. However, they do not have the capability of automatically compensating for build tolerances and wear over the life of the engine as hydraulic lifters do. Means for correcting the thermal pump up problem while retaining the benefits of hydraulic lash adjusters are accordingly desired.